1. Field of the Invention
This invention relates to a light-amount correction dynamic range expanding device. The light-amount correction dynamic range expanding device expands the exposure dynamic range by correcting the light amount of a printer head and is applied to optical color printers each which uses a fluorescent display tube as the light source of a print head to form an image on the photosensitive film.
2. Description of the Related Art
Optical printers such as color video printers utilize fluorescent display tubes each having plural luminous dots as a light source for a print head to form an image on a photosensitive medium coated with a photosensitive agent (e.g. photographic paper or photosensitive film).
Because of variations in brightness between luminous dots of the print head, the printer has a light-amount correction circuit to correct the brightness variation.
FIG. 7 is a block diagram illustrating a conventional light-amount correction circuit. The light-amount dynamic range of the optical printer which uses plural kinds of photosensitive substances with different sensitivities is expanded by increasing the number of exposure gray levels, that is, the bit width of input data and data for exposure process is increased.
For example, data of (n+k) bits in width (e.g. 11 bits when increment k is 3 bits) is used as image data by expanding input data of n bits in width (e.g. 8 bits).
The number of exposure gray levels corresponding to a photosensitive substance in use can be obtained to the input data by referring to the look-up table LUT1 which stores the characteristic curve corresponding to sensitivity correction, that is, density distribution of the photosensitive substance.
Thereafter, in order to correct the light-amount, that is, brightness variations between the luminous dots, the output density characteristic corresponding to an exposure gray level is obtained by referring to the look-up table LUT2 and then is temporarily stored into the temporary memory 31 such as FIFO. Then, the output density characteristic is output to the print head via the gray-scale control circuit (driver circuit) 32 of the print head.
However, even if the I/O data lines at each stage are configured in an (n+k) bit width (e.g. 11 bits in width), the light-amount measuring device merely provides its measurement precision of about 1% and cannot achieve more than 8-bit resolution (1/256=0.4%). The look-up table LUT2 stores correction data to correct variations in brightness of each of dots of the print head. Even when the look-up table LUT2 is prepared based on data measured by the light-amount measuring device in the print head fabrication and the photosensitive medium is exposed with 8-bit resolution (256 levels of gray), the brightness of each luminous dot originally contains an error of about 2.5 levels of gray.
Hence, the precision of the light-amount measuring device cannot be improved even by increasing the number n of the I/O data lines to 8 bits or more in width. The bit increment k increases only the memory capacity of the look-up tables LUT2 and FIFO, without contributing to an increase in quality of the output image.
Moreover, an expensive memory device such as SRAM is used as the look-up table LUT2 which requires a high-speed accessing. Increasing unnecessarily the memory capacity of the look-up table LUT2 results in boosting the fabrication costs and in wasting the memory capacity.